Manufacture of cellulose nitrate



May 7, 1946; L. L. BLYLER I 2,399,620

- MANUFACTURE OF CELLULOSE NITRATE I Filed July 8, 1944 ACTUAL HNO CONTENT s: m A re I23456789I01II2 VISCOSITY CHARACTERISTIC 3 CONTENT A -I N 03 ACTUAL HNO LN LN 2v 4 b 8 I0 I214 16 I8 202224 FILTRATION YIELD L ee L B lgler I ATTORNEY IN V EN TOR.

Patented May 7, 1946 2,399,620 MANUFACTURE OF CELLULOSE NITRATE Lee L. Blyler, Spotswood, N. .I., assignor to E. I.

du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware Application July 8, 1944, Serial No. 544,097

4 Claims. (Cl. 260-220) This invention relates to the preparation of cellulose nitrate from regenerated cellulose and more particularly to the nitration of this form of cellulose whereby a cellulose nitrate of medium viscosity characteristic is obtained.

U. S. Patent 1,997,766 describes a process for preparing cellulose nitrate of low viscosity characteristic from regenerated cellulose, preferably regenerated cellulose sheeting commonly known as Cellophane. As suggested in the patent, this form of cellulose difiers sharply from the cotton linters and wood pulp forms of cellulose commonly used for nitration purposes in that its structure is distinctly non-fibrous, thereby presenting problems both chemical and mechanical that cannot be met by conventional procedures employed in the nitration of the fibrous forms of cellulose. Furthermore, because of the inherently low solution viscosity of the regenerated cellulose caused by the chemical treatments employed in the manufacture of thismaterial, the viscosity characteristic of the cellulose nitrate prepared under normal nitrating conditions without subsequent viscosity reducing treatments such as are commonly employed with cotton linters cellulose nitrate is quite low, that is, on the order of about so-called /2 second (about 3 seconds A. S. T. M. specification D-301-33, formula B) or less. The patent previously referred to describes the manufacture of cellulose nitrate from regenerated cellulose which is characterized by a viscosity below 16 seconds A. S. T. M. specifications D-301- 33, formula C, as secured by direct nitration and without the use of any subsequent treatment such as pressure digestion to reduce viscosity.

The present invention advances the art through the surprising discovery that cellulose nitrates of relatively high viscosity, that is, up to about l2 seconds (A. S. T. M. specification D-301-33, formula A) may be obtained from regenerated cellulose, whichas stated above, has an inherently low solution viscosity, by the use of special nitrating conditions which involve a considerable departure from conventional practice.

This invention has as a primary object the provision of a process for'the nitration of regenerated cellulose, preferably regenerated cellulose sheeting, whereby a cellulose nitrate of relatively high viscosity characteristic is obtained. Another object is the provision of a process for the manufacture of high viscosity cellulose nitrate from regenerated cellulose which is readily soluble in conventional lacquer solvents. Another object is the provision of cellulose nitrate of relatively high viscosity characteristic prepared from regenerated cellulose which is stable. A further object is the provision of cellulose nitrates of high viscosity characteristic prepared from regenerated cellulose having nitrogen contents between about 11% and 12.2%. A further object is the provision of a process for the manufacture of high viscosity cellulose nitrate from regenerated cellulose whereby high filtration yields are obtained. A still further object is the provision of cellulose nitrate having such bulk density and other properties as to make it particularly desirable and acceptable commercially. Other objects will appear as the description of the invention proceeds.

These objects are accomplished in accordance with the present invention by nitrating regenerated cellulose, preferably regenerated cellulose sheeting, previously prepared so that adherence between substantial surface areas is prevented, under critical conditions of nitration temperature and nitric acid content of the nitrating acid mixture. In carrying out the process of the invention, these factors must be carefully controlled to operate at nitration temperatures not exceeding about 30 C. and with a nitric acid content in the nitrating acid mixture of about 40% by weight. The nitrating total of the nitrating acid mixture should also be on the order of about 82% to 86%.

In the drawing the two figures represent curves. In Fig. I the actual nitric acid content of the nitrating acid mixtures is plotted against viscosity characteristic (A. S. T. M. specifications D-301-33, formula A) at the temperatures indicated. In Fig. II, actual nitric acid content of the nitrating acid mixtures is plotted against filtration yields (described more fully hereinafter) at the indicated temperatures.

The following examples are given by way of illustration only and no limitations are intended thereby except as indicated in the appended claims.

Example 1 Regenerated cellulose sheeting (.0009 in thickness) treated to remove softener and moistureproofing coating and mechanically comminuted so as to give small crinkled or crumpled pieces in accordance with the procedure of U. S. Patents 1,997,766 and 2,150,205 was immersed in 36 time its weight of a nitrating acid mixture of the following composition:

. Per cent Actual nitric acid 40.44 Total sulfuric acid 43.86 Nitrosyl sulfuric acid 2.60 Nitrating total 84.30

The bath was agitated for 45 minutes at 20 C. At the completion of the nitration step, the excess acid was removed by centrifuging and the cellulose nitrate stabilized by boiling in acidulated water and then washing in water in the usual manner until acid-free. The final cellulose nitrate had a viscosity characteristic of 3.8 seconds (A. S. T. M. specification D-30133, formula A) and a nitrogen content of 12.04%. The filtration yield which is a measure of solubility and determined in accordance with the procedure hereinafter described was 11.5 grams per square centimeter.

Example 2 Comminuted regenerated cellulose sheeting prepared as described in Example 1 was immersed in 36 times its weight of a nitrating acid mixture of the following composition,

I Per cent Actual nitric acid 50.20 Total sulfuric acid 34.30 Nitrosyl ulfuric acid 2.30 Nitrating total 84.50

The bath containing the acid and the cellulose was agitated for 45 minutes at a temperature of 15 C. After removing the spent acid and stabilizing the resulting cellulose nitrate as in Example 1, it was found that the product had a viscosity of 9.8 seconds (A. S. T. M. specification D-301-33, formula A) and a nitrogen content of 11.87%. The filtration yield of the cellulose nitrate of this example was 18.8' ram's per square centimeter.

Example 3 Regenerated cellulose prepared in accordance with the procedures described in the patents referred to in Example 1 was immersed in 36 times its weight of a nitrating acid mixture having the following composition:

Per cent Actual nitric acid 40.32 Total sulfuric acid 43.28 Nitrosyl sulfuric acid 2.60 Nitrating total 83.60

The bath containing the regenerated cellulose and the nitrating acid was agitated for 45 minutes at a temperature of 30 C. The resulting cellulose nitrate after being freed of spent acid was stabilized in the conventional manner. The viscosity characteristic of the final product was 2.1 seconds (A. S. T. M. specifications D30l-33, formula A) and the nitrogen content was 11.92%. The filtration yield of this cellulose nitrate was 17.1 grams per square centimeter;

Example 4 Regenerated cellulose prepared as in the previou examples was immersed in 20 times its weight of a nitrating acid mixture of the following composition:

Per cent Actual nitric acid 50.00 Total sulfuric acid 35.60 Nitrosyl sulfuric acid 3.43 Nitrating total 85.60

The nitrating bath containing the cellulose was. agitated for 48 minutes at a temperature of 20 C. after which the Spent acid was removed by centrifuging and the cellulose nitrate stabilized by the customary boiling in acidulated water followed by additional treatment in a series of fresh water washes in accordance with the usual practice. The viscosity characteristic of the final product was 4.3 seconds and the nitrogen content 11.70%. The filtration yield of the cellulose I nitrate of this example was 17.7 grams per square centimeter. It will be noted that in this example the ratio of nitrating acid to cellulose was considerably lower than in the other examples and similarly lower than is the usual practice with cotton linters cellulose. This departure from conventional ratio which is permitted by the special form of cellulose and under the nitrating conditions of the present invention is of importance because of the desirable economic advantages thus afforded.

Example 5 Regenerated cellulose prepared as in the revious examples was immersed in 36 times its weight of a nitrating acid mixture having the following composition:

Per cent 40 Actual nitric acid 54.88

Total sulfuric acid 29.82 Nitrosyl sulfuric acid 2.15 Nitrating total 84.70

Regenerated cellulose prepared as in the previous examples was immersed in 36 times its weight of a nitrating' acid mixture having the following composition:

- Per cent Actual nitric acid 45.73 Total sulfuric acid 39.77 Nitrosyl sulfuric acid 2.45 Nitrating totaL. -1 85.50

The bath containing the acid and cellulose was agitated for 45 minutes at a temperature of 0-5 C. After freeing the resulting cellulose nitrate of spent acid, it was stabilized in the usual manner. The viscosity characteristic of the final product was 6.7 seconds (A. S. T. M. specification D-301-33, formula A) and the nitrogen content was 12.06%. The filtration yield of this cellulose nitrate was 10.7 grams per square centimeter.

assume 3 Solutions of the examples prepared in accordance with formula A of A. S. T. M. specification D-301-33 gave a color and appearance equal or superior to cellulose nitrate solutions prepared from cellulose nitrate of the same viscosity characteristicprepared from. purified cotton linters. The solutions from Examples 2, 3, 4 and 5 were particularly outstanding in the absence of grain.

As previously stated, the filtration yield of the'cellulose nitrate is an excellent measure of the solubility of the cellulose nitrate in solvents employed in preparing solutions of the products for commercial utilization in lacquers, enamels, adhesives, etc., particularly in clear lacquers. This determination was made by preparing a, solution of the cellulose nitrate in formula B, A. S. .T. M. specification D30l33, which was then filtered through filter paper under pressure of about 50 pounds per square inch. The result is ex pressed in grams of exuded bone dry cellulose nitrate per square centimeter of filter area.

The apparatus in which this test is carried out consists of a filtration tube fabricated from a brass cylinder inches long having an inside diameter of 1% inches. The tube is provided with threaded brass caps on each end. The test isoperated with the tube in a vertical position. The bottom cap is perforated and holds a round piece of felt approximately /8 inch thick. On top of thefelt is placed a Carl Schleicher 520-B filter paper and the felt and paper are held in place by a metal ring of known inside area. This assembly is then screwed tightly on to the bottom end of the tube. The tube is then filled with the cellulose nitrate solution to be tested and air pressure at 50 pounds per square inch, maintained at this pressure by an automatic regulator, supplied to the space above the solution. This pressure is applied until the flow of material through the filter stops. The amount of filtrate per square centimeter of filtering area is then determined. A cellulose nitrate which yields a filtrate containing more than 5 grams of bone dry cellulose nitrate per square centimeter is considered satisfactory.

In obtaining the products of the present invention, it has been found that the nitric acid content in the nitrating acid mixture and the temperature at which the nitration is carried out are controlling and quite critical in the successful operation of the process. Acceptable cellulose nitrates of viscosity characteristic between about 1.5 seconds and about 11.5 seconds (A. S. T. M. specification D-30l-33, formula A) may be obtained using nitrating acid mixtures containing between about 40 and 55% nitric acid by weight and nitrating at temperatures between 0 C. and 30 C. The preferred operating conditions, whereby optimum results are secured particularly with respect to solubilities, include a nitric acid content in the nitrating acid mixture of between about 40% and 50% by weight and a nitrating temperature of between about C. and 30 C.

Cellulose nitrates of nitrogen contents between about 11.0% and 12.2% can be successfully prepared at the viscosity characteristics within the range suggested above in accordance with the procedures and nitrating conditions of the present invention.

Although in the examples, the thickness of the regenerated cellulose sheeting was about .0009", it is possible to operate the invention successfully with thicknesses of .0013" or up to about .0018" or even higher although at these greater thickmasses i.'0018"+) some difficulty is experienced in obtaining complete acid penetration.

The time of. nitration may be varied from 15 minutesto 5 or more hours depending upon other nitration conditions. A range between about /2 hour and 1 hour is generally preferred.

The c'uprammonium viscosity or so-called solution viscosity of the regenerated cellulose is about 2 compared to water as 1. The method of determming this viscosity is a follows: Dudley pipettes of c. 0. capacity, giving drainage times of 33-35 seconds for water at 25 C. were selected and standardized using distilled water. During the tests, the pipettes were maintained at 25 C. by a surrounding water jacket. On measuring the solution viscosity of a sample of regenerated cellulose, 3.05 grams of cupric hydroxide, 5 grams of regenerated cellulose and 225 c, c. of ammonium hydroxide (sp. gr., 0.96) at 25 C. were introduced into an 8 oz. wide mouth bottle, the bottle closed by means of acork, and the mixture tumbled overnight. The solution was then placed in a water bath maintained at 25 C. for one hour after which it was drawn into the pipette and the time of draining of 100 c. 0. measured with a stop watch. The specific solution viscosity is calculated by reference to the draining time of water.

The curves in the attached drawing clearly illustrate the effects secured in varying the controlling factors of nitration temperature and nitric acid content in the nitrating acid mixtures in obtaining the higher viscosity and superior solubility characteristics, thus affording a very desirable product not heretofore available. Figure'l consists of a series of curves for different nitration temperatures showing the effect on the viscosity characteristic of the cellulose nitrates of the present invention prepared from nitrating acid mixtures containing varying amounts of nitric acid. The trend toward higher viscosities with higher nitric acid content acid mixtures and a lower nitrating temperature i strikingly illustrated. Figure 2 shows the effect on the important propert of solubility expressed as filtration yield of increasing the nitric acid content in the nitrating acid mixtures.

U. S. Patent 1,997,766 provides for important volume commercial use of a formerly waste material since prior to the patented process, practically all of the scrap regenerated cellulose sheeting was destroyed by burning. The products of the patent consisting of low viscosity cellulose, that is, of viscosity below one second (A. S. T. M. specification D-301-33, formula A) and for present large scale production, of viscosity of /z second or less, which products have found widespread use in lacquers and enamels where excellent quality, uniformity and economic advantage particularly in shipping have been afforded. The regenerated cellulose used in the patented process and in the present invention as previously stated has a low solution viscosity (cuprammonium viscosity) as a result of the chemical and mechanical treatments employed in the manufacture of this material. This property, therefore, permits the manufacture of low viscosit cellulose nitrate by direct nitration without the use of subsequent viscosity reduction treatment, for example, pressure digestion, such as is commonly used for the reduction in viscosity of cellulose nitrate prepared from the conventional cotton linters cellulose.

Because of the low inherent solution viscosity of the regenerated cellulose, the manufacture of the relatively high viscosity cellulose nitrate of the present invention appeared to be impossible and it was only after exhaustiveresearch investi-' gation that the practical procedures for attaining the desired objective were evolved. The new process, therefore, greatly extends the commercial usefulness of a waste material for the preparation of coating and adhesive compositions. where higher viscosity cellulose nitrate is required for greater flexibilit and toughness or for other reasons.

An advantage of prime importance resulting from the present invention resides in the making available of a new cellulose nitrate of medium viscosity obtained by direct nitration, avoiding the necessity of any subsequent treatment for the reduction in viscosity characteristic. The new product is characterized by marked-uniformity and excellent solubility in cellulose. nitrate solvents. Other important advantages over cellulose nitrates of similar viscosity characteristic prepared from fibrous cellulose such as cotton linters include, lower spentacid losses, more rapid dissolution in lacquer solvents, less iron and other contamination from nitrating. acids and stabilizing and wash waters, greater resistance to degradation during nitration and lower bulking value. This latter property is of real economic value from the standpoint of shipping and storage since the product of the present invention has a bulking value of about 27.2 pounds per cubic foot whereas the bulking value of commercial fibrous nitrocellulose is but 18.4 pounds per cubic foot.

It is apparent that many widely diiferent embodiments of the invention may be made with out departing from the spirit and scope thereof and, therefore, it is not intended to be limited except as indicated in the appended claims.

I claim:

1. In the process of preparing cellulose nitrate with a viscosity characteristic of between about 1.5 seconds and 11.5 seconds determined in accordancewith A. S. T. M. specification D-301-33, formula A, from regenerated cellulose in the form of small crumpled pieces of a thickness of less than .0018 inch, the step of nitrating said cellulose at a temperature of between about 0 C. and 30 C. with a nitrating acid mixture consisting essentially of nitric and sulphuric acids, the nitrating total of which is between 82% and 86% of which between about and 55% by weight is nitric acid. I

2. Process of claim 1 in which the nitrating temperature is between about 15 C. and 30 C.

3. Process of claim 1 in which the nitric acid content in the nitrating acid mixture is between about 40% and by weight.

4. In the process of preparing cellulose nitrate with a viscosity characteristic of about 4 seconds determined in accordance with A. S. T. M. specification D-301-33, formula A, from regenerated cellulose sheeting in the form of small crumpled pieces of a thickness of less than about .0018 inch, the step of nitrating said cellulose at a temperature of 20C. with a nitrating acid consisting of nitric and sulphuric acids, the nitrating total of which is between 82% and 86% of which about 40% by weight is nitric acid.

LEE L. BLYLER. 

